In the event that there is a small break in the piping for the coolant and consequently loss of coolant in a conventional pressurized water reactor, the pressurized water evaporates and overheats the core, and thus it forms a steam and gas bubble in the upper head. To remove this steam and gas bubble, it has been suggested to place an external vent in the upper-most part of the vessel head. A tube is connected to the vent to lead the steam away from the vessel head. However, placing such a vent in the head portion presents three difficulties. Firstly, the vessel head is fitted with many control rods which make access to the vent extremely difficult. Secondly, should the tube connected to the vent break, because of the high pressure within the vessel, the broken tube will "whip" which may damage the control rod driving mechanisms to cause an enormously difficult problem. Thirdly, the installation of such an external vent assembly in the vessel head increases the time in refueling since the vent must be disassembled and then reassembled to return to normal operation. For these reasons, commercial operators of nuclear reactors have disliked the idea of placing a vent at the top of the vessel head. From the above description, it is clear that a means for safely and conveniently venting a pressurized water reactor without placing an external vent on the vessel head of the reactor is needed.